The present invention is generally directed to a system and method for displaying surface information. The images of the surfaces displayed are typically contained within the interior regions of solid bodies which are examined by computed tomographic (CT) x-ray systems or by magnetic resonance (MR) imaging systems either of which is capable of generating three dimensional arrays of data representative of one or more physical properties at various locations within a three dimensional volume. More particularly, the present invention is directed to a system and method for the display of medical images so as to obtain representations of internal bodily structures. The images generated in the practice of the present invention provide three dimensional data for examination by physicians, radiologists and other medical practitioners.
In conventional x-ray systems, a two dimensional shadow image is created based upon the different absorption characteristics of bone and soft tissues. A great improvement on the conventional x-ray systems as a diagnostic tool has been provided by computed tomographic systems which have been developed over the last ten years or so. These so called CT systems are x-ray based and initially were used to produce single two dimensional views depicting transverse slices of a body, object, or patient being investigated. Three dimensional information was thereafter gleaned from CT scan data by generating data for a number of contiguous slices and using the inferential abilities of the radiologist to suggest a three dimensional representation for the various internal organs. In one embodiment of the present invention, shaded and contoured three dimensional images are generated from the three dimensional array of data generated by a sequence of such contiguous CT scans. In the same way, the newer MR imaging technology is also capable of generating three dimensional arrays of data representing physical properties of interior bodily organs. Moreover, MR systems have the capability to better discriminate between various tissue types, not just bone and soft tissue. MR imaging systems are also capable of generating physiological data rather than just image data. However, whether MR or CT systems are employed, the data has been made available only as a sequence of the slices and systems have not generally been available which provide true three dimensional images.
In the present invention, three dimensional data generated either by a CT scanning system or by an MR imaging system may be displayed and analyzed in a plurality of ways so as to produce on a display screen or other device, a multitude of anatomical features which are selectable at the viewer's choice. In the system and method of the present invention, the data used to produce the three dimensional images is typically acquired once and then used and re-used to generate information and to display images at the option of the viewer. The viewer is provided with the option of selecting one or more threshhold values which determine, for example, whether or not bone surfaces as opposed to brain surface tissue is to be displayed. The viewer or operator of the present system can also select the appropriate viewing angle and can, at will, selectively ignore segments of the data generated in order to provide cross sectional views through any desired plane. Moreover, the viewing angle is selectable and it is possible to generate a sequence of images and display them sequentially to provide the medical practitioner with interior views of solid surfaces in a truly three dimensional manner from any desired viewing angle with the further capability of being able to construct a view through any plane or slice. Again, it is pointed out that for many purposes, an almost infinite variety of meaningful images can be created from only a single set of MR or CT scan slice data arrays. Certainly though, if the objective of the medical investigation is the study of internal anatomic variations as a function of time, then it meaningful to produce a sequence of three dimensional data arrays indexed by time. The system and method of the present invention provide the medical practitioners, and surgeons in particular, with the ability to plan detailed and complicated surgical procedures using totally non-invasive diagnostic methods. The images generated by the present invention can only be described as truly dramatic and show every evidence of being as great an improvement in the medical imaging arts as computed axial tomography and magnetic resonance imaging.
While the system and method of the present invention will undoubtedly find its greatest utilization in the analysis and display of tomographic x-ray and magnetic resonance imaging data, the system of the present invention is equally applicable to systems employing ultrasound, positron emission tomography, emission computed tomography and multi-modality imaging. Moreover, while the present invention is particularly applicable to the construction of medical images, it is also pointed out that the system and method of the present invention is applicable to the display of interior three dimensional surface structures for any system which is capable of generating three dimensional data arrays in which signal patterns are present which represent the value of at least one physical property associated with points in a solid body.
A particular advantage of the present invention is its ability to provide the medical practitioner with the means to perform interactive functions in real time. Systems which do not permit interactive use suffer a significant disadvantage since a real time display methodology is required for optimal human interaction with the system, particularly in the case of a surgeon planning a difficult procedure. For example, in transplant surgery, it is often difficult to ascertain beforehand the precise shape or size of a body cavity which is to receive an implant. This is true whether or not the implant comprises human tissue or a mechanical device. It is therefore seen that it would be very important for a surgeon to be able to display the cavity in question on a screen in three dimensional form and be able to rotate it and section it at will, before any invasive procedure is undertaken. It is also important to such medical practitioners that the images generated are sharp and exhibit excellent contrast. The images generated should also depict surface texture wherever this is possible.
The display of three dimensional graphic images on a cathode ray tube (CRT) screen has principally been driven by the goals and directions of computer aided design (CAD) and computer aided manufacturing (CAM). Systems have been developed for displaying solid bodies and for manipulating images in various fashions to create solid models for manufactured parts and for rotating and viewing these parts from a multiplicity of directions. In particular, CAD/CAM systems have been developed which accept data in two basic formats. In a wire-frame display format, the display processor is provided with a sequence or list of three dimensional points representative of the end points of line segments. These line segments are joined to represent various surface structures. An advantage of these wire frame images is the ability to rapidly rotate the image about various axes to obtain different views. In the other format, the raster format, an image is generated on a screen or other display device as a collection of individual picture elements (pixels) whose intensity and color are determinative of the image displayed. In the raster based format, an electron beam is typically made to scan across a phosphorous screen in horizontal lines which are sequentially "painted" on the screen. The system and method of the present invention are more closely related to the raster based format and representation than the so-called vector based method. The vector based/polygonal approaches described in application Ser. No. 741,390 filed June 5, 1985 and application Ser. No. 741,391 filed June 5, 1985 by one or more of the present inventors, said applications being assigned to the same assignee, are particularly accurate in their representation of surface detail. In these two patent applications, the polygonal resolution was in general not related to the resolution of the screen on which the image was displayed. However, a particular advantage of the present invention is that, by subdivision and interpolation, 3-D images may be generated with a resolution which closely matches the resolution of the screen. This resolution is typically measured in dots or pixels per inch. Alternatively, screen resolution may be expressed in terms of dot pitch with typical high resolution screens having a dot pitch of approximately 0.3 dots per millimeter in current devices. The raster format is particularly useful for displaying images which are more closely related to images as perceived by the human eye, as opposed to wire frame images.
Related work in the field of displaying three dimensional images has been carried out by Gabor Herman who has employed a method in which each adjacent volume element is analyzed and quantized to discrete zero and one values. Surface approximations are made only by considering cube faces and surface normal information can only be partially reconstructed because of the quantization step that is performed. The resulting method produces low resolution images.
Meagher, working for Phoenix Data Systems, has employed a method of octree coding in which the three dimensional data array is subdivided into eight regions and each region is subdivided until individual volume elements are formed. Regions not containing surfaces are not subdivided. However, this method requires special purpose hardware. While the images are crisp, individual volume elements produce a quantized artifact that is not observed in smooth tissues such as bone. Other methods for displaying three dimensional data are, for example, described in U.S. Pat. No. 4,475,104 issued Oct. 2, 1984 in the name of Tsu Y. Shen. This patent appears to disclose a three dimensional display system which incorporates a depth buffer to provide separate 3D information as part of the mechanism for generating appropriate shading values.
Accordingly, it is seen that it is an object of the present invention to provide a system and method for the display of three dimensional information.
It is a further object of the present invention to provide a display system for use in conjunction with CT scanners, ultrasound devices, MR imaging systems, and any and all other systems capable of generating three dimensional data representative of one or more physical properties within a body to be studied.
It is yet another object of the present invention to provide a graphic system for medical images which is capable of interactive use and yet at the same time produces high quality images providing textural, shading, and other visual clues to the user.
It is yet another object of the present invention to provide a three dimensional graphics display system which is compatible with current CAD/CAM systems.
Another object of the present invention is the generation and display of three dimensional raster format based information.
Still another object of the present invention is to maximize the information contained in a three dimensional data array for the purpose of surface representation.
It is also an object of the present invention to provide a system and method which is readily fabricatable in conventional electronic hardware, especially that used in CAD/CAM systems.
It is yet another object of the present invention to provide medical practitioners with the ability to emulate surgical procedures graphically prior to undertaking invasive measures.
Additionally, it is an object of the present invention to provide a plurality of three dimensional surface views from a single set of collective data.
Lastly, but not limited hereto, it is an object of the present invention to provide a system and method for the display of three dimensional images of internal surface structures in such a way that the specific viewing angle and cross sectional viewing plane may be selected by the user in an interactive manner.